Towards PFAS-free high temperature proton exchange membrane fuel cell cathodes: Evaluating PEEK as an alternative to PTFE

Kurzfassung

Polytetrafluoroethylene (PTFE) is used in various applications throughout the renewable energy sector: in wind turbine bearings, as back sheets for solar panels or as separators in batteries. In particular, in the catalyst layer (CL) of a high temperature proton exchange membrane (HT-PEM) fuel cell PTFE is dominant due to its hydrophobic properties while being chemically and thermically resistant to the working environment of the FC. In search for a suitable replacement for PTFE with respect to a possible per- and polyfluoroalkyl substances (PFAS) ban, polyether ether ketone (PEEK) based CLs are intensively studied in this work as a possible substitute for PTFE. PEEK is commercially available and a production would be scalable. It exhibits a high thermal and chemical resistance suitable for the HT-PEM environment. Loadings of 20, 30 and 40 wt. % PEEK were tested against PTFE loadings of 0, 10, 20 and 30 wt. %. The following measurements were used: Micro X-ray fluorescence analysis (micro-XRF), scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX), contact angle analysis, inductively coupled plasma mass spectrometry (ICP-MS), nano computed tomography (n-CT) and electrochemical measurements. Even though the hydrophobicity of the PEEK based CL is not comparable to PTFE based electrodes, the electrochemical half-cell investigation of the oxygen reduction reaction (ORR) showed promising results. PEEK based electrodes in general lead to a better performance compared to a CL without polymer or a non-optimal PTFE loading such as 10 wt. % PTFE. The electrode with 30 wt. % PEEK delivered a potential E at - 0.5 A mgPt−1 only 8 % lower than the commercial PTFE based reference. The incorporation of PEEK into HT-PEM cathodes achieves this by compared to PTFE increased open porosity and decreased agglomeration the catalyst material.